我是 Django 新手，不明白什么是 pk 或 slug。到底是怎么回事？模型.py：class School(models.Model):    name = models.CharField(max_length=256)    principal = models.CharField(max_length=256)    location = models.CharField(max_length=256)    def __str__(self):        return self.name模板頁面：<a class="navbar-brand" href="{% url 'basic_app:list'%}">Schools</a>網址.py：path('',views.SchoolDetailView.as_view(),name='list'),視圖.py：class SchoolDetailView(DetailView):    context_object_name = 'school_detail'    model = models.School    template_name = 'basic_app/school_detail.html'